Presently Al2O3:C phosphor has acquired the status of an acclaimed phosphor for optically stimulated luminescence based radiation dosimetry and is being extensively used in different radiation dosimetry related applications. The established approach for the large scale production of such dosimetric grade Al2O3:C is based on single crystal growth of Al2O3 from its molten state carried out in a strongly reducing environment of graphite i.e. carbon by means of Czochralski technique. The feed material for this process is corundum (Alumina) crystal grown from alumina powder by another process of crystal growth called Verneuil method. The said method for synthesis of Al2O3:C thus involves two stages. In the stage one alumina crystals are grown by Verneuil method and the stage two in which these alumina crystals are again re-melted and subjected to Czochralski technique of crystal growth in the reducing environment of graphite to finally yield Al2O3:C. Each one of them demands a different type of equipments.
In Czochralski technique, the crystals are grown by inserting a small seed Al2O3 crystal of an appropriate orientation mounted on the tip of a rotating shaft into a crucible containing molten alumina. The shaft rotates in counterclockwise direction and the crucible which contains molten alumina rotates in clockwise direction. The shaft is slowly lowered into the crucible until it just touches the surface of the molten alumina then the shaft along with the seed is slowly withdrawn from the melt in a vertically upward direction. This seed being at a relatively lower temperature, allows the melt to solidify on it. Since the seed itself being a crystal, wherein the material exists in a highly ordered form, the material which gets subsequently accumulated on this seed too solidifies in the form of a crystal, provided the rate of cooling is slow enough. The slow cooling rates are achieved by a very gradual and controlled withdrawal of seed from the melt. Thus, this approach towards synthesis of dosimetric grade Al2O3:C material essentially involves high precision and therefore an inherently costly equipments and associated overheads.
Another factor that contributes to the overheads is the cost of pre-processing of raw material alumina powder into corundum (Alumina) crystals grown by Verneuil method to yield the feed material on which this Czochralski technique relies upon and amounts to an additional contribution to the overheads.
Most importantly, the Czochralski technique suffers from a major disadvantage in that the composition of a crystal so grown does not remain constant along the direction of its growth. Besides this, segregation of the dopants is another known disadvantage of this technique. As dosimetric performance of a material critically depends on nature, concentration and uniformity in the distribution of defects the drawbacks of Czochralski method mentioned earlier result into undesirable variations in dosimetric behavior of crystals even though their growth conditions differ only marginally, Moreover, even the samples that are cut out from different regions of the very same single crystal too are known to exhibit (undesirable) deviations in their dosimetric performances.
K. P. Muthe et al., Journal of Luminescence 128 (2008) 445-450, provides a process for the preparation of said dosimetric grade Al2O3:C from polycrystalline alumina powder without involving the Czochralski technique. Thus the synthesis technique as disclosed in this literature, advantageously avoids the limitation relating to the Czochralski method by introducing a simple carbon-doped alumina samples preparation process involving melting of polycrystalline alumina powder in graphite environment. However the synthesis technique has some inherent limitation which limits its relevance in the large scale synthesis of dosimetric grade Al2O3:C, such as, the low dosimetric efficiency reported in the said literature (0.65 times of that commercially available reference α-Al2O3:C crystal) due to inadequate concentration and inconsistent distribution of dosimetrically relevant defects. The result that OSL efficiency of even the best samples produced using the method reported in the said literature was only 0.5 times of the commercially available reference Al2O3:C material produced using the usual crystal growth route can be interpreted to imply that this method cannot produce dosimetrically useful defects in sufficient concentration and/or the distribution of defects produced through this method is non-uniform.
U.S. Pat. No. 6,414,324 teaches a method of preparing a luminescent detecting material such as anion-deficient Al2O3 for use in UV dosimetry which utilizes photo transferred luminescence wherein the detecting material has a set of shallow dosimetry traps for trapping electronic charge carriers, which are thermally released upon heating to a first temperature, and a set of deep traps for trapping electronic charge carriers, which charge carriers are released upon heating to a second temperature. The detecting material is prepared by irradiating the detecting material to fill the shallow and deep traps with charge carriers, heating the material to release charge carriers from the shallow traps, and then cooling the material.
U.S. Pat. No. 6,811,607 provides aluminum oxide crystalline materials including dopants and oxygen vacancy defects and methods of making such crystalline materials. The crystalline material of the present invention has particular utility in optical data storage applications.
Both these above said US arts rely on the usual method of crystal growth which has inherent limitation as discussed in the previous.
Thus, based on the aforesaid discussions and interpretations of the available experimental observations it is apparent that there has been a need for the development of a process routine adapted to synthesis large scale of dosimetric grade Al2O3 without involving any high precision and therefore an inherently costly instrumentation and expensive feed material. More particularly, the new method should be a straightforward process to synthesize a polycrystalline material with sufficient grain size adapted to serve as acceptable dosimetric material in radiation dosimetry applications wherein the expensive approach based on lengthy single crystal growth is indeed not necessary.